Category: Archeology

The ways archaeologists use uranium, volcanoes, and trapped electrons for dating

We’ve covered several dating methods on StoneAgeMan, like dendrochronology and radiocarbon dating, but there are still more ways archaeologists can tell how old a site or artifact is. Three of the most clever techniques utilize uranium, volcanoes, and trapped electrons.

We’ll touch on each of them below, and provide examples of places where archaeologists have used them.

Uranium-series Dating

Did you know that scientists can tell how old a cave painting is by dating the rocks on top of or underneath it? Uranium-series (U-series) dating is the tool they use to accomplish such a feat, and here’s how it works:  

Like radiocarbon dating, Uranium-series (U-series) methods rely on radioactive decay. They track the breakdown of two isotopes of uranium – 235U and 238U – into a series of “daughter isotopes.”

238U, for example, decays into 234U and thorium-230 (230Th). 230Th then continues to break down, until it transforms into a stable isotope of lead called 206Pb.

Scientists have learned how long it takes the above isotopes to decay: 238U has a half life (the time it takes for half of a sample to decay) of 4.47 billion years, 234U 245,500 years, and 230Th 75,584 years.

Therefore, by measuring how much of the parent and daughter isotopes of uranium remain in a sample, scientists can determine how old it is.

When Would You Use U-Series Dating?

The two main types of U-series dating are U-Th (uranium-thorium) and U-Pb (uranium-lead) dating. Since the latter is for objects that are over 1 million years old, archaeologists mostly use U-Th dating.

U-Th dating is most effective on samples that are between 50,000 – 500,000 years old. It works on shells, teeth, bones, and more.

One fascinating application for U-series dating is determining the ages of cave paintings.

Here are two studies, one from China and the other from Spain, in which the authors collected samples of calcite (calcium-based rocks) that were either underneath a cave painting, or that had formed on top of one.

Dating the calcites underneath a painting allowed the archaeologists to determine its maximum age, because the painting couldn’t be older than the surface it was painted on. Conversely, dating calcites that had formed on top of a painting gave archaeologists a minimum age for it.

U-series dating also works on bones, and these scientists used it to date a controversial skull that was found in Ethiopia. Image by sbox from Pixabay.

Dating with Volcanic Rock: Potassium-Argon Dating

We humans seem to be drawn to geologically-active areas that want to kill us, like California, Japan, and the slopes of Mount Vesuvius.

Fortunately, our ancestors were no different. Scientists are now using a dating method called potassium-argon dating – which works exclusively on volcanic rocks – to learn about our earliest forebears.  

Potassium-argon (K-Ar) is another radiometric dating method, meaning that it relies on radioactive decay. More specifically, it requires the breakdown of potassium-40 (40K).

Volcanic rocks don’t contain the element argon when they’re formed, because the heat of the eruption forces it out of them. But over time, 40K in the rocks decays into argon-40 (40Ar).

The creation of volcanic rocks. Image by skeeze from Pixabay.  

As with U-series and radiocarbon dating, scientists know how quickly 40K decays. They can also measure how much 40Ar is in a volcanic rock, and thereby learn its age.

When archaeological remains lie above or below layers of volcanic rock, scientists can employ K-Ar dating to calculate maximum and/or minimum ages for those remains.

When Would You Use K-Ar Dating?

K-Ar dating comes with two catches.

The first is that 40K decays slowly: it takes about 1.3 billion years for half of the 40K in a sample to change into 40Ar. Consequently, K-Ar dating typically only works on rocks that are over 80,000 years old.

The second catch is that volcanic rocks aren’t found everywhere.

Thankfully, some crucial archaeological sites lie in volcanic areas. This includes Tanzania’s Olduvai Gorge, which boasts the most extensive record of human evolution of any site.

Scientists have found the fossils of multiple species of early hominids in Olduvai Gorge, along with their stone tools. They’ve also used K-Ar dating extensively in this location, and at other sites in East Africa.

Olduvai Gorge, sometimes called “The Cradle of Mankind” (“The Cradle of Humankind” is a different site). Image by Harvey Barrison, found on Wikimedia. CC BY-SA 2.0  

Luminescence Dating

The final two dating methods we describe here utilize electrons that have become trapped in crystalline minerals. They’re called thermoluminescence (TL) and optically-stimulated luminescence (OSL) dating.

Thermoluminescence Dating

Pottery is one of the most common types of artifacts that archaeologists find. But how do you date it? If you’re only interested in relative dating, then techniques like typology and seriation would suffice.

However, if you want to attach a specific year to pottery or similar items, then TL dating will help you do so.  

Certain materials – such as flint, pottery, and bricks – have “holes” that loose electrons can get stuck in.

For instance, if a pottery sherd was buried in the ground, then over time radiation in the surrounding environment would free some of the sherd’s electrons – a portion of which would become trapped in defects within the pottery.

Broken pieces of pottery are called “sherds.” Black Burnished Ware Pottery Sherds (exterior) by Frank Basford. CC BY-SA 2.0 

Heating an object to a high enough temperature (listed as 500º Celsius in most sources) will release its trapped electrons.

To continue the example from above, when ancient people fired our pottery sherd to harden it, they reset its “electron clock” to zero.

As such, if scientists could measure how much energy (electrons) had become trapped in the sherd, and if they knew the level of radiation it was exposed to, then they’d know how long ago it was fired.

That’s exactly how TL dating works.

Scientists take a sample, heat it, and record the energy that it emits as light (luminescence). In addition, they place devices that measure radiation at the spot where the sample was found.

With these two pieces of information, scientists can calculate when a sample’s electron clock was last reset.

It’s possible for natural fires to reset artifacts’ electron clocks, too. That’s why archaeologists need to carefully examine the contexts of all their finds, so they can identify complicating factors like wildfires. Image by Alexas Fotos from Pixabay.  

Optically-Stimulated Luminescence Dating

OSL dating functions in much the same way as TL dating, with one key difference: rather than heating a sample to release its trapped electrons, scientists expose it to light. This mainly works on materials that contain quartz or feldspar.

When Would You Use Luminescence Dating?

TL dating’s sweet spot is from 40,000 – 200,000 years ago. Archaeologists have used it successfully in several cases, but one example comes from a study in Malaysia, where they utilized TL dating to determine the ages of ancient bricks.

By contrast, OSL dating’s ideal time period varies depending on the mineral being tested. 

OSL dating is reliable for quartz up to 200,000 years ago, and potentially longer for feldspar. Unfortunately, electrons sometimes leak out of feldspar grains, generating results that are too young.

Professor Nick Barton and his colleagues conducted a noteworthy OSL-based study in Morocco. There, they drew on OSL dating to ascertain the ages of a series of artifacts that came from the Aterian industry.

What Barton and his coauthors learned was that these Aterian artifacts made it to Morocco before archaeologists thought possible, meaning that technological development spread faster throughout North Africa than scientists realized.

Wrapping Up

By now, you’ve probably realized that archaeologists can use a wide array of methods to figure out how old an object is.

These methods require different types of materials, work best for specific ages, and have their own strengths and weaknesses.

For this reason, the best practice is to use multiple dating methods at the same site. This will produce the most accurate results, because the various techniques will compensate for one another’s blind spots.

Sources on Dating Methods

The following list of articles were of great use for this article and provide a wonderful background on dating methods. 

How Trees Tell Time: Dendrochronology

Did you know that trees are some of the best ‘clocks’ on the planet? You may have heard that you can determine a tree’s age by counting its rings, but if you count the rings on lots of trees you can date archaeological sites going back tens of thousands of years!

Image by MabelAmber from Pixabay.

Previously on StoneAgeMan, we covered relative and radiocarbon dating methods. While those are the most common dating techniques, there’s another method that can date archaeological remains to an exact calendar year, and make radiocarbon results more accurate. It’s called dendrochronology, or tree-ring dating.

Basic Premises of Dendrochronology

Trees produce rings each year that they grow. Trees grow more during wet years, producing wide rings, and less during dry years, leaving narrow rings. Since no two years have the same precipitation levels, this generates unique patterns of wide-to-narrow rings. However, because trees from the same region receive similar amounts of moisture, they develop ring patterns that are close enough to be synced up.

Dendrochronology as an Absolute Dating Method

It was an astronomer named Andrew Ellicott (A. E.) Douglass who first used tree rings to date archaeological sites. He began studying tree rings in the Southwestern United States in 1901 to see if they’d reflect sun spot activity, and he soon realized their usefulness for archaeology.

Douglass, an anthropologist named Clark Wissler, and several other researchers worked with the indigenous peoples of the Southwest to collect samples from as many trees as they could. As they obtained these samples, Douglass and his colleagues were able to arrange them in chronological order using a method called crossdating.

Crossdating

Crossdating starts with a tree of known age, so that dendrochronologists can match each of its rings to a specific year. Dendrochronologists then produce a “skeleton plot” by lining up a piece of graph paper with a cross-section of the tree, or a narrow core that they’ve drilled out of it, and recording all of the narrow rings.

Dendrochronologists can then use this skeleton plot to date an older tree whose lifespan overlapped with the younger one.

With the help of the skeleton plot, dendrochronologists find which pattern of narrow rings matches that of the younger, dated tree: allowing them to identify the years in which both trees were alive. By counting backwards from the last overlapping year, dendrochronologists can then determine the age of the older tree. They can also construct a skeleton plot of the tree, and then date even older specimens.

An example of a skeleton plot. Image from Page 280 of “Brigham Young University Science Bulletin” (1955) from Internet Archive Book Images. No known copyright restrictions.

Douglass and his teammates employed crossdating to establish a “master sequence” of tree-ring patterns in the U.S. Southwest going back to 700 AD, which dendrochronologists have now extended to 6700 BC.

What this means for archaeologists is that if they find a piece of timber in a Southwestern archaeological site, and if they can match its tree-ring pattern with the master sequence, then they can attach a precise calendar year to when that site was active (provided the tree wasn’t long dead before people used it). The same goes for any other region in which dendrochronologists have developed master sequences.

Drawbacks

The main problem with dendrochronology – at least as an absolute dating method – is that it’s geographically limited.

Dendrochronology is only useful in climates that are arid enough to produce distinct ring patterns. In moist regions, like the Eastern U.S., ring widths don’t vary much.

Another problem is that not all tree species generate ring patterns that are both clear and consistent enough to be used for dating. Certain varieties of trees, such as willows, can have erratic ring patterns that confuse researchers. By contrast, oaks are among the most reliable species for dating.

Not all tree species produce clear, distinctive ring patterns that are suitable for dating. Tree Rings by Bill Kasman. Public Domain

Since tree rings are sensitive to both climate and species, master sequences have only been produced for a few regions and types of trees. These include an Irish oak chronology that extends to 5300 BC, an oak chronology in Germany that stretches to 8500 BC, and a California bristlecone pine chronology in the Southwestern U.S. that dates to 6700 BC.

Despite the above limitations, dendrochronology is critical worldwide for its ability to calibrate radiocarbon results.

Dendrochronology for Radiocarbon Calibration

Recall that radiocarbon dating works by measuring the amount of carbon-14 (14C) left in an organic sample, and then checking this against the background 14C level in the earth’s atmosphere.

Unfortunately, the 14C content in the earth’s atmosphere has fluctuated over time, and scientists need to be able to account for these changes to calibrate radiocarbon results. The data for such calibration comes from tree rings.

Tree rings preserve 14C well. When working with a tree that has been dated, scientists also know the year that corresponds with each ring. This means that if they measure the amount of 14C left in a dated tree ring, they can use the known decay rate of 14C to figure out how much of it was in the earth’s atmosphere during that year.

An example of a radiocarbon calibration curve. The blue line shows the “raw” radiocarbon results, and the red shows calibrated results that account for carbon-14 fluctuations over time. Image found on Wikimedia. Public Domain.

Tree rings are powerful calibration tools up to 12,600 years ago, beyond which dendrochronologists haven’t established master sequences. Scientists need to use other dating methods to check radiocarbon results that exceed 12,600 years old, which we’ll discuss in the future.

Key Take-Aways

  • In temperate climates that don’t get too much rain, tree rings can date archaeological remains to exact calendar years.
  • Dendrochronologists use crossdating to organize tree rings into master sequences that can cover thousands of years.
  • Even in regions where dendrochronology doesn’t work as an absolute dating method, tree rings are the main tools for calibrating radiocarbon results up to 12,600 years old.

Sources

Becker, B. (1993). An 11,000-year German oak and pine dendrochronology for radiocarbon calibration. Radiocarbon, 35(1), 201-213.

Beta Analytic Testing Laboratory. (2016, May 5). Radiocarbon tree-ring calibration.

Cook, E. R., & Pederson, N. (2011). Uncertainty, emergence, and statistics in dendrochronology. In Hughes M., Swetnam T., & Diaz H. (Eds.). Dendroclimatology, developments in paleoenvironmental research (11th ed.). Dordrecht: Springer.

Crow Canyon Archaeological Center. (2019). Dendrochronology.

Douglas, A. E. (1929). The secret of the Southwest solved by talkative tree rings. The National Geographic Magazine.

Douglas, A. E. (1941). Crossdating in dendrochronology. Journal of Forestry, 39(10), 825-831.

Ferguson, C. W. (1970). Concepts and techniques of dendrochronology. In R. Berger (Ed.). Scientific methods in medieval archaeology (183-200). Berkeley, Los Angeles, and London: University of California Press.

Kromer, B. (2009). Radiocarbon and dendrochronology. Dendrochronologia, 27, 15-19.

Mason, M. Dendrochronology: What tree rings tell us about past and present. EnvironmentalScience.org.

Nash, S. E. (2017, November 8). How archaeologists uncover history with trees. SAPIENS.

Oxford Radiocarbon Accelerator Unit. Radiocarbon calibration.

Renfrew, C., & Bahn, P. (2015). Archaeology essentials (3rd ed.). London: Thames & Hudson.

Schields, B. (2015, November 18). Be a dendrochronologist! Project Archaeology.

Time Team America. (2013, January 30). Dendrochronology: How tree-ring dating reveals human roots. PBS.

UCAR Center for Science Education. (2014). Tree rings (dendrochronology).

Archaeological Dating Methods

Part 1: Relative and Radiocarbon Dating

A question I frequently hear about archaeology is, “How do archaeologists know how old something is?” Indeed, determining when an artifact or feature was made is a key part of learning about past civilizations.

There are several dating methods that help archaeologists figure out how old objects are. In fact, there are so many that it would be impossible to describe them all in one article. Hence, this post will discuss some of the most widely-used dating methods – stratigraphy, typology, seriation, and radiocarbon dating – and we will cover the rest in subsequent articles.

Relative Dating Methods

There are two overarching classes of dating methods: relative and absolute. Relative dating methods cannot determine the exact age of an object, but only which finds are older or younger than others. The most important relative dating method relies on a site’s stratigraphy.

Stratigraphy

When excavating an archaeological site, you can literally see the layers of dirt and debris that have accumulated over time. These layers are known as a site’s stratigraphy, and the law of superposition, first popularized by Sir William Matthew Flinders Petrie, states that the oldest stratigraphic layers are at the bottom. Thus, objects found near the top of a site are probably younger than the ones further down – unless something (like a burrowing animal) moved the items after burial.

Here is an example of a site profile showing the location’s stratigraphy. View of Fell’s Cave Stratigraphy by University of Iowa Press. CC BY-SA 3.0

Typology and Seriation

Other relative dating methods depend on examining the physical characteristics of archaeological finds. In a given culture – or amongst connected cultures – artifacts with similar styles (typologies) tend to be popular at specific times. This leads to the principle that “like goes with like,” or that objects that look the same were probably made during the same periods.

While studying the typologies of archaeological remains allows researchers to figure out which objects are close in age, seriation helps them track cultural trends.

Let me explain seriation through a hypothetical example. For instance, assume that archaeologists working in a given site have found the remains of many ceramic bowls that have distinctive, horizontal bands along their lips. By recording the stratigraphic layers in which they uncovered the bowls, the archaeologists noticed that they found a few bowls in the oldest layers of their site, lots of them in the middle layers, and then just a handful in the most recent layers.

Many assemblages, or groups of similar artifacts, go through this pattern: they emerge, rise to their peak in popularity, and then fade away. This pattern is known as the “battleship curve,” because it looks like a battleship when viewed from above (from the side???).

Here is a seriation diagram involving different types and time periods of tombstones at Old City Cemetery in Vancouver, Washington, where the data is laid out in the classic battleship curve pattern. Public domain photo, credits to the National Park Service.

By tracking the battleship curves of different assemblages of artifacts, archaeologists can observe the evolution of past technologies over time.

Absolute Dating Methods

Absolute dating methods can attach specific years to archaeological finds. More specifically, they provide ranges of possible years, because no absolute dating method is exact – with the possible exception of dendrochronology (tree ring dating). Each absolute dating method also requires a different kind of sample, which means that not every method can be used on each site. The most commonly-used absolute dating method needs carbon-based samples.

Radiocarbon Dating

Radiocarbon dating was developed in the 1950s by chemist Willard F. Libby. It revolves around the element carbon-14 (14C): an isotope of carbon that is produced when the sun’s energy interacts with nitrogen atoms in the earth’s atmosphere.  

Plants absorb 14C through photosynthesis, and animals consume it by eating plants or other animals. When plants and animals die they stop acquiring 14C. Since 14C is mildly radioactive and naturally decays into 12C, plants and animals slowly lose their 14C after they stop obtaining it.

Fortunately for archaeologists, 14C decays at a uniform rate. It takes 5730 years for half of the 14C in a given sample to decay, so scientists say that its half-life is 5730 years. When an organism is alive and acquiring carbon, its 14C content reaches an equilibrium with its environment. Thus, by measuring how much 14C is left in a dead plant or animal, and then checking this against the background 14C level in the atmosphere, scientists can discern how long ago that organism died.

Radiocarbon dating is an effective way to date carbon-based artifacts (e.g. charcoal, animal bones, seeds, etc.) that are up to 50,000 years old. Radiocarbon dates are usually expressed as a range of years BP, or before present (the year 1950), and might look something like 2000 ± 100 BP. Once archaeologists know the age of an organic sample, they can then apply that date to other archaeological remains found in the same stratigraphic layer.

Charcoal was a relatively common find on the excavations I worked on. Photo downloaded from Needpix.com.    

Modern radiocarbon techniques can produce accurate results with small sample sizes, but radiocarbon dating still has drawbacks. The main issue is contamination.

If an archaeological sample comes into contact with another carbon-based object, it can alter the amount of 14C in that sample, making any subsequent radiocarbon dates inaccurate. Related to contamination is the reservoir effect.

14C mixes more slowly in water than it does air. Consequently, radiocarbon dates obtained from marine samples (any organism that lived in the sea) frequently appear older than the object actually is. This is a serious problem in regions like the Arctic, where indigenous peoples have long relied on marine mammals – including seals – to survive. Archaeological sites in the Arctic can be so inundated with seal oil that it becomes difficult to generate accurate radiocarbon dates, because many of the samples suffer from the marine reservoir effect.

Furthermore, radiocarbon dates must be calibrated. The amount of 14C in the earth’s atmosphere has fluctuated over time, which means that radiocarbon results do not translate directly into calendar years unless they are synced with reliable data about past 14C levels. This information, along with a powerful absolute dating method in its own right, comes from tree rings.

We will continue discussing archaeological dating methods here on StoneAgeMan with dendrochronology, or tree ring dating.

Sources

Here is How Archaeologists Excavate Sites

Previously on StoneAgeMan, we discussed how archaeologists find sites. But, once a promising site has been identified, how do archaeologists excavate it? Here, we will detail the process, which more often than not, involves a lot more than just digging.

First, no two excavations are the same: how one unearths a site will depend on a host of factors that might include the area’s terrain; funding, time and crew-size constraints; and the guiding research questions. However, there are two overarching excavation styles called the Wheeler box-grid method and open-area excavation.

Wheeler Box-Grid Method vs. Open-Area Excavation

Wheeler box-grid

The Wheeler box-grid method involves dividing an archaeological site into a series of orderly squares with uniform spaces – or balks – between them. Teams of workers will then remove the dirt within the squares, and as they dig, the balks become walls that reveal the stratigraphy of the site: the layers of soil that have built up over time. Archaeologists can then use this stratigraphy to help date the artifacts and features that they find.

The Wheeler box-grid method is still popular in regions like South Asia, but it has fallen out of favor in other parts of the world due to its rigidity. A more flexible and widely-used excavation method is called open-area excavation.

Open-area excavation

Open-area excavation still consists of digging in pre-designated squares – called units – but they are not always laid out in a formal grid pattern. Units in the open-area method can be spread out or close together, depending on the site characteristics and the main research questions. Another difference between open-area excavation and the Wheeler box-grid method is that the former style does not always leave formal balks between units. Thus, open-area excavation requires precise measurements and documentation to record the all-important context of a site.

This archaeologist is definitely not using the Wheeler box-grid method. The Excavation by Odense Bys Museer. CC BY-SA 2.0.

Working on an Excavation

To give some idea of what it is like to work on an archaeological dig, I would like to describe my excavation experiences – all of which have followed the open-area method.

The first step on all the excavations I worked on was to define our units. We used tape measures to carefully delineate exact squares whose dimensions were usually 1 x 1 metre, but sometimes 1 x 1.5 metres. My teammates and I then placed stakes at each corner of the units and tied them together with brightly-colored string, double-checking our measurements as we went.

A team of archaeologists excavating a site; note the perfectly-shaped walls. IMG_4681 by ChattOconeeNF. CC BY 2.0.

Once our units had been outlined, it was time to dig – by which I mean scrape.

Very rarely did I dig with shovels on my excavations, aside from removing the topmost layers of soil in an agricultural field that had been mixed-up by plowing (known as the plow zone). Almost invariably, what we actually did was take small garden trowels and turn them on their sides. We then slowly scraped the soil away with the sides of our trowels, gripping them where the handles met the shovel portions.

As we removed the dirt, we scooped it into buckets. We had metal screens set up close to our units, and when the buckets were full we dumped them on the screens and sifted through their contents to make sure we had not missed any artifacts. It can be difficult to distinguish broken pieces of pottery from rocks, and the same goes for stone tools – called lithics. This meant that my colleagues and I had to work together to avoid accidentally throwing away artifacts.

We placed any artifacts we found into bags on which we wrote the site name, the unit, and the level of the unit in which we had found the artifacts (e.g. “Unit 1 Level B”). In Belize, we had separate bags for ceramic, lithic, organic, and other types of artifacts. This detailed recording is essential, because when you excavate a site you destroy its context. Thus, without detailed records, it would be difficult to learn anything from an excavation.

Archaeologists placing a ceramic artifact into a carefully-labeled bag. Archaeological Field School by U.S. Army Corps of Engineers. Public Domain.

Record-Keeping on Excavations

Apart from labeling all of the artifacts we found while screening, my teammates and I also documented the locations of all the features and noteworthy artifacts that we found inside our units. This was not easy.

When we uncovered a feature or a diagnostic artifact – like a projectile point that was intact enough to determine which group of people may have made it – we had to map it. To do this, we first dangled a plum bob from a string at a 90º angle above the object. We then lined up a tape measure with the string, and recorded the depth of the object in centimeters.

In addition to measuring the depth of features and artifacts, we also recorded how far they were from their units’ walls. We thus knew how far down important finds were, and where they were located horizontally. We used these measurements, along with notes about the changes in soil types within our units to draw detailed side-profiles and plan-maps of our units.

Furthermore, we took meticulous photographs of each unit at several stages during their excavations.

The photographs taken during archaeological excavations need to be labeled too. Grand Archaeology – IMG_0566 by Grand Canyon National Park. CC BY 2.0

These measurements, profiles, and photographs were a pain to generate in the field – especially in the jungle – but without them our excavations would have had little scientific value: archaeologists need these notes to interpret the finds they make in the field.

Key Take-Aways

Hopefully this short article has provided some idea of how archaeologists excavate sites. The most important points to remember are:

  • Much of the “digging” in archaeology is comprised of carefully scraping away dirt with a trowel.
  • Everything – including the locations of artifacts and features, the profiles and layouts of units, and each layer of soil – must be recorded during a dig. These measurements, drawings, and photos preserve the context of a site after it has been destroyed, and this context helps archaeologists generate knowledge about humanity’s past.

If you want to learn more about archaeological excavations, contact your state, provincial, or local archaeological societies and ask about volunteering – the best way to learn about archaeology is to do it! (is there a link we can send people to? Maybe a list of state archeological societies?

Sources

McMillon, B. (1991). The archaeology handbook: A field manual and resource guide. New York, NY: John Wiley & Sons, Inc.

Renfrew, C., & Bahn, P. (2015). Archaeology essentials (3rd Ed.). London: Thames & Hudson.

How do Archaeologists Locate Sites?

I get this question a lot: “How do archaeologists know where to dig?” The answer is that there are many ways to find archaeological sites. These include talking to locals, learning what to look for through experience, employing modern technology, and walking surveys. I will briefly describe what each of those methods entail, to give you a better idea of how archaeologists locate sites.

Local Information and Experience

The people who live in or frequent an area may already know where to look for archaeological sites. For instance, this past summer I volunteered on a dig in northern Ohio where the landowners had been finding artifacts since the 1920s – decades before the first archaeological digs.

Thus, speaking with locals can be one way to pinpoint sites. In the United States and Canada, there are many avocational (non-professional) archaeological societies comprised of people who are experts on their region’s past, and these can be excellent groups to consult with.

Experienced archaeologists can also be surprisingly good at spotting landscape features that might indicate the presence of sites.

I got a taste of this during my archaeology field school in Belize. While hiking through the jungle, we would often pass low hills or mounds that looked natural to me, but which the more advanced group members instantly recognized as buried Mayan structures.

Mayan temples in Mexico.
While in Belize, I saw buried temples that looked just like natural hills. Unlike the temples above, they were completed covered by soil and natural vegetation. Image by Jon Toy from Pixabay.

Technology

Technology is revolutionizing archaeology. Techniques like aerial and satellite photography, along with Light Detection and Ranging (LiDAR), allow archaeologists to locate sites from above.

Aerial photography can be especially useful over cropland. Buried features can change the richness of the soil above them, causing the plants in their vicinity to grow more robustly or sparsely than the surrounding vegetation. This forms patterns – or crop-marks – that are visible from the air.

In addition, satellites can spot features that are hard to notice at ground level. This was the case in 1992, when a satellite detected ancient footpaths in the deserts of Oman. By tracing these footpaths to the location where they converged, archaeologists re-discovered the lost city of Ubar.

LiDAR goes a step further than standard aerial photography. It works by emitting lasers, which, when mounted to an overhead aircraft, bounce off the ground and back to a receiver on the aircraft. LiDAR devices measure how long it takes the lasers to return, and this data can be used to generate detailed models of the terrain.

In other words, LiDAR allows archaeologists to see through dense vegetation, thereby revealing the locations of sites that might otherwise take years to find.

Temple I and the Main Plaza at the Mayan site of Tikal.

In 2018, LiDAR scans revealed that the Mayan city of Tikal in Guatemala is substantially larger than previously realized. Seen here is Temple I and part of the Main Plaza at Tikal. Image by ickandgak from Pixabay.

I was able to witness the benefits of LiDAR firsthand while in Belize. The team I was working with had recently conducted LiDAR scans of the area surrounding their sites, and they learned that many more structures lay deeper in the jungle. Our most experienced team members immediately organized an expedition to those structures, during which they had many, jaguar-filled adventures.

There are also several ground-based technologies that can detect buried objects. These include magnetic and electromagnetic sensing devices, various types of probes, machines that measure electric resistivity (the ease or difficulty with which an electric current moves through a substance), and ground-penetrating radar (GPR).

The above technologies, both airborne and ground-based, can be extremely useful for detecting hidden features and artifacts, but they all have limitations.

For example, GPR devices project radar waves into the earth, which then reflect off the objects they hit and are measured by a receiver on the GPR machine – forming three-dimensional images of the underground environment. However, wet and clayey soils absorb radar waves, limiting GPR’s effectiveness on such terrain.

Due to these limitations, along with time and funding constraints, walking surveys will never go out of style.

Walking Surveys

Walking surveys are tried-and-true methods to find archaeological sites.

A team of archaeologists stands in a straight line along a rope, preparing for a survey.
A team of archaeologists prepares for a survey. Archaeological site of Martignana in the Orme Valley near Montespertoli/Italy by Roman Rural Landscapes | University of Vienna. CC BY 2.0.

Walking surveys can be unsystematic or systematic. In the former scenario, teams of surveyors simply walk around and look for artifacts on the surface of the ground. Systematic surveys are more complicated than their unsystematic brethren, but they are also less biased.

One way to conduct a systematic survey is through transects: pre-determined, straight lines in which team members walk – no matter what – while recording each artifact that they find. This is a good way to determine where the most promising locations are to excavate, but many study areas are too large to be covered entirely by transects. Thus, another option is to use a variety of sampling techniques to divide the landscape into squares, which can then be surveyed.

What to Remember About Archeology Surveys

In summary, there are many ways that archaeologists locate sites. Talking to locals is often a good place to start, as they may already know where people have been finding artifacts or features. Experience also makes it easier to identify good places to dig, as do a host of technological devices.

Modern technology is becoming increasingly valuable for archaeology. Airborne devices like LiDAR are proving indispensable for finding new sites, or in expanding our knowledge of existing ones. Ground-based remote-sensing technologies keep improving as well, and can greatly assist the surveying process.

Finally, teams of trained surveyors – either walking in transects or working in sample squares – will always be an indispensable part of archaeology.

Sources:

Ford, A., & Horn, S. (2014). Above and below the Maya forest. Science, 361(6409).

Herbich, T. (2012). Geophysical methods and landscape archaeology. Egyptian Archaeology.

McMillon, B. (1991). The archaeology handbook: A field manual and resource guide. New York, NY: John Wiley & Sons, Inc.

Kline, Eric. (2017). Three stones make a wall: The story of archaeology. Princeton, New Jersey: Princeton University Press.

Pîrnău, R. G., Mihu-Pintilie, A., Bodi, G., Asăndulesei, A., & Niacşu, L. (2014). Ground-penetrating radar as noninvasive method used in soil science and archaeology. Soil Forming Factors and Processes from the Temperate Zone, 13, 15-31.

Redmond, B. G., & Scanlan, B. L. (2010). A report of archaeological investigations at the Heckleman Site (33Er14): 2009 season. Cleveland Museum of Natural History Archaeological Research Reports, No. 158.

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Smekalova, T. N., Yatsishina, E. B., Garipov, A. S., Pasumanskii, A. E., Ketsko, R. S., & Chudin, A. V. (2016). Natural science methods in field archaeology, with the case study of Crimea. Crystallography Reports, 61(4).

The Three-Age System: What it is and Why it Matters

The terms Stone, Bronze, and Iron Age are used frequently. What not everyone might realize, though, is that these labels refer to an actual “thing” called the Three-Age System. This was one of prehistoric archaeology’s first dating systems, and it revolutionized the field in the 19th Century.

Background

As with most breakthroughs, the Three-Age System was the culmination of several advancements in knowledge. For many centuries, the dominant understandings of prehistory in the West were based on the Bible. Hence, the world could only be 6,000 years old, and the art of metallurgy (smelting and shaping metals) had been around since humanity’s earliest days, because the Bible said so.

When Europeans found stone artifacts, they thought they were “ceraunia” that were created by lightning striking the ground. Late in the 17th Century, following contact with indigenous peoples in the Americas and South Pacific, European scholars began to realize that these ceraunia were crafted by people who did not know how to work metal. This led to much debate.

How could Europeans have learned metallurgy from Noah’s descendants – as the book of Genesis described – and then forgotten it? Gradually, antiquarians (early archaeologists), naturalists, and geologists began to acknowledge that both Earth and humanity were much older than they had imagined.

The stage was now set for a paradigm shift, which would be instigated by Christian Jürgensen Thomsen (1789 – 1865)

C.J. Thomsen

Thomsen took over as the secretary for Denmark’s Royal Commission for the Preservation of Antiquities in 1816. He was not an academic, but he kept up with scientific discussions and knew the Royal Commission’s collection of artifacts better than anyone. Thus, in 1819 he opened the Royal Museum of Nordic Antiquities.

Thomsen was forced to relocate his museum in 1832 to accommodate the large crowds it was attracting. This required him to reorganize the museum’s artifacts, and he chose to arrange them according to the chronological ages he had come to believe in.

According to Thomsen, first came stone tools and weapons, then bronze; and, finally, iron. He published this dating scheme in 1837 in an essay titled “Brief Outlook on Monuments and Antiquities from the Nordic Past.”

Thomsen was not the first person to propose a Stone, Bronze, Iron Age sequence for humanity’s past, but his proposal gained more traction than previous ones. This was partly due to timing, and the fact that Thomsen’s apprentice, J. J. Worsae, later uncovered empirical evidence to support Thomsen’s theory.

While it was not immediately accepted by everyone, Thomsen’s Three-Age System changed Western conceptualizations of prehistory forever. But what are the Stone, Bronze, and Iron Ages?

Stone Age

The Stone Age is the period of time before a society had widespread access to metal tools and weapons. Britannica says that the oldest human tools date to 3.3 million years ago, meaning the Stone Age may have begun before Homo sapiens existed.

However, most sources claim that the Stone Age lasted from 250,000,000 BCE to around 3,000 BCE. It is often broken into the Paleolithic, Mesolithic, and Neolithic Periods, which describe increasing levels of technological and social complexity.

In the Paleolithic Period, which corresponds to the massive environmental fluctuations of the ice ages, everyone was hunter-gatherers. The Earth warmed as the Neolithic approached, which is the period when humans developed agriculture and animal domestication.

The usual story is that Middle Eastern societies first invented agriculture and animal domestication in 7,000 – 6,000 BCE, and then this knowledge spread throughout Eurasia (Europe and Asia). While this is likely true to some extent, archaeologists are finding evidence that farming sprang up independently in parts of Europe at around the same time, raising the possibility that multiple peoples may have learned to domesticate plants and animals at similar times.

The Mesolithic Period represents the transition between hunter-gatherer (Paleolithic) and agricultural (Neolithic) life.

Bronze Age

A society entered the Bronze Age when it learned to produce and utilize bronze tools and weapons. This first began in about 3,300 BCE in the Middle Eastern civilization of Sumer.

Ancient peoples learned to make bronze by mixing copper – which on its own is not any better than stone – with small amounts of tin. This produced a hard metal that made excellent tools and weapons.

A great deal of cultural change also took place during the Bronze Age. This was the age during which humans invented the wheel and written language, when King Hammurabi developed an advanced legal system, when the Classic Greeks laid the foundations for modern democracy, and the glory days of Ancient Egypt.

Even larger changes began in 1,200 BCE, when people in the Middle East and Southeastern Europe learned to smelt iron.

Iron Age

Iron, while not tougher than Bronze on its own, is significantly more common than copper and tin. Hence, the invention of iron metallurgy led to the first mass-production of metal tools and weapons. 

Nearly all of Eurasia had entered the Iron Age by 500 BCE, and most of the world is currently in this age.

Caveats

The Three-Age System contains important nuances. 

First, the dates of the three ages are relative. Second, Thomsen created the Three-Age System to help him categorize Nordic artifacts; and, consequently, it most readily applies to Eurasia. Technological change did not occur at the same rate in Sub-Saharan Africa and the Americas, which were isolated by the Sahara Desert and the oceans, respectively.

The third nuance is that the Three-Age System is not a progressionist view of history: societies do not necessarily get “better” as they shift from one age to the next, they simply become more complex.

Wrapping it up

Despite the above caveats, the Three-Age System provides a convenient framework for quickly describing the evolution of human technologies and social systems. Since it focuses on tool-making materials, which archaeologists can empirically study, it is also fairly objective.

Today, however, we have ways to more precisely date prehistoric artifacts. Those techniques may be the topics of future articles here on StoneAgeMan. And, as a reminder, we have a StoneAge Man youtube channel.

References

Archaeology Soup. (2011). Aspects of Archaeology: The Three (and a bit) Age System.

Bronze Age. (2019). History.com.

Eskildsen, K. (2012). The Language of Objects: Christian Jürgensen Thomsen’s science of the pastIsis, 103(1), 24-53.

FerreroNutella. (2005). The three ages of human antiquityEverything2.

Fitzpatrick-Matthews, K. (2007). The Three-Age SystemBad Archaeology.

Goodrum, M. R. (2002). The meaning of ceraunia: archaeology, natural history and the interpretation of prehistoric stone artefacts in the eighteenth century. BJHS, 35, 255-269.

Heizer, R. (1962). The background of Thomsen’s Three-Age SystemTechnology and Culture, 3(3), 259-266.

Hirst, K. K. (2019). Three-Age System – Categorizing European prehistoryThoughtCo.

Høst-Madsen, L., & Harnow, H. (2012). “Historical archaeology and archaeological practice in Denmark.” In Across the North Sea, Later Historical Archaeology in Britain and Denmark, c. 1500-2000 AD.

Iron Age. (2019). Encyclopaedia Britannica.

Major Periods in World HistoryBiography Online.

Movius, H. L., Gimbutas, M., Braidwood, R. J., Pittioni, R., Adams, R. M., & Keesing, F. M. (2018). Stone AgeEncyclopaedia Britannica.  

Rowley-Conwy, P. (2004). The Three Age system in English: New translations of the founding documentsBulletin of the History of Archaeology14(1), 4-15.The Stone, Bronze, and Iron Ages. (2019). Essential Humanities.

Archaeology: The Science of Uncovering the Past

Few sciences have been more mythologized than archaeology. Whether in films like Indiana Jones, video games like Tomb Raider, or television series like Expedition Unknown, archaeology is billed as a high-octane treasure hunt.

That is not what the field’s actually about.

What is archaeology?

Archaeology’s objective isn’t to pursue bizarre conspiracies or magical objects, but to uncover knowledge about humanity’s past. Guided by specific research questions and rigorous methods, contemporary archaeologists study the material remains of past civilizations to generate theories about what historic peoples were doing. They then publish their interpretations in academic journals, where other archaeologists can critique and learn from them. That’s how the science progresses.

Archaeologists primarily examine two classes of materials. Artifacts are transportable objects that were made, altered, or used by humans. They can include fragments of chipped stone, ceramic sherds, animal bones with cut marks, etc. Features, by contrast, cannot be easily moved. The remains of buildings, campfire sites, and cavities left by wooden posts (known as “post molds”) belong to this category, along with many other items.

This piece of volcanic glass, or obsidian, might be an artifact shaped by humans. Note the bulge at the bottom, which could be a percussion bulb from when someone knocked this piece of obsidian loose from a larger chunk in a process called flintknapping. Image by Carla Burke from Pixabay.

Artifacts and features can hold a wealth of data about how people lived, and this data is the real treasure that archaeologists seek.

It hasn’t always been this way, however.

History

People have likely been digging up the remains of prior civilizations for millennia. Despite this, the modern, Western practice of archaeology officially began in 1709 with the excavation of Herculaneum: one of the Roman cities buried by the 79 CE eruption of Mount Vesuvius.

As author Eric Kline describes in Three Stones Make a Wall, these digs were little more than looting, since the “archaeologists” working on the site had no interest in learning about the peoples whose remains they were uncovering. Instead, they were collecting rare objects and ancient statues for personal gain.

In other cases, early archaeologists were primarily concerned with chasing legends. The most well-known of these legend hunters was Heinrich Schliemann, a wealthy, amateur archaeologist who was obsessed with finding the city of Troy from Homer’s Iliad. Even though he eventually succeeded, Schliemann committed a number of transgressions that would be unthinkable by archaeologists today.

The most severe of these was that he destroyed much of the city he sought. 

The site that Schliemann was excavating, called Hissarlik, was in modern Turkey. Hissarlik contained several Troys stacked on top of each other, the oldest versions of the city being towards the bottom.

Schliemann was convinced that the second-oldest Troy, Troy II, was the one from The Iliad. Unfortunately, that Troy was one thousand years too old to be the one Schliemann was looking for, which he eventually realized.

Troy VI and VII correspond to the time frame from The Iliad, making them the best candidates for the “right” Troy. Unfortunately, Schliemann dug a trench straight through them, dismantling a palace that may have held valuable data.

Modern archaeologists work as meticulously as they do to avoid making mistakes like Schliemann’s.

The Present

Archaeology is much different today than it was in Schliemann’s time. 

For one thing, archaeologists no longer dig straight through ancient sites in search of legends. Rather, they carefully scrape away the dirt from their units – purposefully-designated squares in which to dig – while painstakingly documenting noteworthy finds. Archaeologists also make detailed maps and profiles of their units, replete with photographs that can be surprisingly difficult to take. This is because the context of an archaeological find can transmit more information than the artifact or feature itself.

Archaeology has also grown more diverse as the field has expanded, developing subdivisions designated prehistoric and historic archaeology. The former concentrates on civilizations that existed before the advent of written language, whereas historic archaeology involves societies that had a writing system. Thus, historic archaeologists can sometimes use written documents to aid their quests for knowledge.

There is also underwater archaeology, experimental archaeology, indigenous archaeology, cultural-resource management, classical archaeology, industrial archaeology, and more. Archaeologists can work for museums, universities, private companies, government agencies, and media, making for a variety of career paths.

Underwater archaeologists have made many valuable finds by investigating underwater shipwrecks. Image by Romero Chaves from Pixabay

The growth of archaeology benefits everyone, because it’s a vital field.

Why Archaeology Matters

Why go through all the trouble of scientifically excavating a site, analyzing the findings in a lab, and then subjecting oneself to the torments of publication? There are many reasons, but perhaps the best comes from two of the earliest modern archaeological sites: Herculaneum and Pompeii.

Though the inhabitants of these cities met a tragic end, Vesuvius preserved Herculaneum and Pompeii remarkably well. These sites give us extraordinary glimpses into the past, that, as Kline explains, allow us to see that these ancient peoples were remarkably similar to ourselves.

Pompeii, shown here with Mount Vesuvius in the background, is one of the most important archaeological sites in the world. Image by Michael Swanson from Pixabay.

By highlighting both the diversity of human cultures through the ages, and those aspects of the human experience that transcend time, archaeology teaches us about who we are as a species. 

Sources: